Device formed by selective deposition of refractory metal of less than 300 Angstroms of thickness

ABSTRACT

A method and the device produced by the method of selective refractory metal growth/deposition on exposed silicon, but not on the field oxide is disclosed. The method includes preconditioning a wafer in a DHF dip followed by the steps of 1) selectively depositing a refractory metal on the exposed surfaces of the silicon substrate by reacting a refractory metal halide with the exposed surfaces of said silicon substrate; 2) limiting silicon substrate consumption by reacting the refractory metal halide with a silicon containing gas; and 3) further increasing the refractory metal thickness by reacting the refractory metal halide with hydrogen. Through an adequate pretreatment and selection of the parameters of 1) temperature; 2) pressure; 3) time; 4) flow and 5) flow ratio during each of the deposition steps, this invention adequately addresses the difficulties of uneven n+ versus p+ (source/drain) growth, deep consumption/encroachment by the refractory metal into silicon regions (e.g., worm holes), poor adhesion, uncontrolled selectivity and uneven morphology.

This application is a division of application Ser. No. 08/753,128, filedNov. 20, 1996 U.S. Pat. No. 5,807,788, issued Sep. 15, 1998.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to deposition techniques for forming metalregions on semiconductor substrates, and more particularly, to theselective deposition of refractory metals on semiconductor substratesand the integrated circuit (IC) devices formed thereby.

2. Description of the Related Art

The continued miniaturization of integrated circuits has brought aboutan increasing need to fabricate source/drain/gate structures and contactmetallurgy with acceptable electrical characteristics. In recent years,much effort has been focused on the use of metal silicides to fulfillthis need. However, as device dimensions become even smaller, bothvertically and horizontally, suicides lose their attractiveness. Theintrinsic resistivity of the suicides is high compared to metals. Inaddition, silicide consumption (vertical growth/deposition of thesilicide into the substrate) and encroachment (horizontalgrowth/deposition of the silicide into the substrate) is too large forthe contact structures proposed for future devices. By way of example,in the case of formation of cobalt disilicide (CoSi₂), approximately 3.4Angstroms (A) of silicon substrate are consumed for every A of CoSi₂formed. Other suicides used in the source/drain regions of the FieldEffect Transistors (FETs) usually consume about 500 A of silicon fromthe junction. This requires forming deep source/drain regions using twophotomask steps and a thick source drain spacer.

Refractory metals have been investigated as possible alternatives tosilicides. Their low resistivity and relatively high temperaturestability makes the refractory metals attractive. In addition, therecent development of selective chemical vapor deposition (CVD)processes, have made refractory metals, such as tungsten (W) andmolybdenum (Mo), prime candidates to replace suicides. However,processes used in depositing refractory metals still have limitationswhich have not been adequately addressed.

One difficulty is uneven growth/deposition rates upon n⁺ and p⁺ (e.g.,source/drain) diffusion regions. This is undesirable. For both contactresistance and sheet resistance, there should be no difference betweenresistance in the n+ and p+ diffusion regions.

Another difficulty is deep consumption/encroachment by the refractorymetal into the silicon regions. Selective refractory metal films, suchas W, similar to silicides, consume Si during their formation. As devicedimensions decrease, the depth of the junction is reduced andconsumption/encroachment must be minimized to prevent damage to thedevice. Shallow junctions are especially sensitive to these phenomenaand high junction leakage can occur if consumption and encroachment arenot minimized. The desired Si consumption should be limited to less than100 A.

Poor adhesion is another difficulty. Refractory metal films, such as W,are known to not adhere well to Si substrates due to the presence ofnative oxide. This may result in peeling of the W film during subsequentthermal cycling after further processing of the wafer.

Another difficulty arises when selectivity of the growth/deposition ofthe refractory metal is uncontrolled. An ideal selectively depositedmetal grows only on exposed Si and not on the surrounding oxide/nitride.When the refractory metal grows on the exposed areas that are notsilicon, shorting of adjacent components can occur. In addition,encroachment problems may occur due to the refractory metal penetratingalong nearby silicon dioxide/silicon interfaces.

The morphology of the selective refractory metal film should be smoothat the Si interface. If the surface is rough, spiking may occur into theSi resulting in device failure. Having a smoother refractory metal filmprovides better contact resistance. In addition, a smooth surface ismore effectively covered by thin metal liner layers that are required asadhesion layers for deposition of subsequent metal layers. Smooth filmsare also more readily integrated with photolithographic processes.

In accordance with one particular CVD technique, tungsten (W) isdeposited on the surface areas of a silicon substrate by placing thesubstrate in a CVD reactor and heating the substrate. Tungstenhexafluoride (WF₆) and an inert carrier gas such as argon (Ar) ornitrogen (N₂) are then fed into the reactor and the WF₆ reacts with thesilicon substrate in accordance with the following: ##EQU1## Thedeposition of W will stop and in order to deposit additional material,hydrogen (H₂) is added to the WF₆ and carrier gas. The WF₆ will reactwith the H₂ to deposit the desired additional W in accordance with thefollowing: ##EQU2##

Another deposition technique is disclosed in U.S. Pat. No. 5,202,287 toJoshi et al., assigned to International Business Machines Corporationand entitled Method of a Two Step Selective Deposition of RefractoryMetals Utilizing SiH₄ Reduction and H₂ Reduction. This two step processincludes a first deposition step, such as silane (SiH₄) reduction of atungsten hexaflouride (WF₆) in accordance with the following: ##EQU3##followed by a further selective deposition process of reacting WF₆ withhydrogen: ##EQU4##

The use of the above processes to selectively deposit tungsten for VeryLarge Scale Integration VLSI application has been limited by problemsinherent in the deposition process. The results of each deposition stepare dependent upon five parameters: 1) temperature; 2) pressure; 3)time; and 4) flows and 5) flow ratio. The deposition steps and theparameters used in the deposition steps of the heretofore mentionedprocesses have not adequately addressed the following: a). uneven n+versus p+ (source/drain) growth/deposition; b). deepconsumption/encroachment by the refractory metal into silicon regions(e.g., worm holes); c). poor adhesion; d). uncontrolled selectivity ande). uneven morphology.

SUMMARY OF THE INVENTION

This invention is directed to a method and an integrated circuitproduced by the method for selectively depositing a refractory metallayer on exposed silicon surfaces of a silicon substrate utilizing athree step deposition process. The steps of the deposition processinclude: selectively depositing a refractory metal on the exposedsurfaces of the silicon substrate by reacting a refractory metal halidewith the exposed surfaces of said silicon substrate; limiting siliconsubstrate consumption by reacting the refractory metal halide with asilicon containing gas; and further increasing the refractory metalthickness by reacting the refractory metal halide with hydrogen.

Through an adequate pretreatment and selection of the parameters of 1)temperature; 2) pressure; 3) time; 4) flow and 5) flow ratio during eachof the deposition steps, this invention adequately addresses thedifficulties of uneven n+ versus p+ (source/drain) growth, deepconsumption/encroachment by the refractory metal into silicon regions(e.g., worm holes), poor adhesion, uncontrolled selectivity and unevenmorphology.

This invention also has the cost advantage of depositing a selectiverefractory metal utilizing blanket deposition equipment (that typicallyoperates in the 1 Torr or higher pressure regime). In most selectiveprocesses, costly equipment (using turbo and cryopumps to operate in the1 m Torr pressure regime) must be utilized. In addition, the cost oftraditional selective tungsten tooling is increased due to the use of aclustered dry etch step used before selective tungsten deposition. Thisinvention only requires a single step of preconditioning the siliconsubstrate by a dilute hydrofluoric acid (DHF) dip to thereby eliminatenative oxides. A vacuum break after the DHF dip is allowed.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred exemplary embodiment of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements, and:

FIG. 1 is a diagrammatic view of a W CVD reactor of a preferredembodiment of the present invention;

FIG. 2 is a diagrammatic view of a preferred embodiment of the presentinvention prior to selective W deposition;

FIG. 3 is a diagrammatic view of a preferred embodiment of the presentinvention following selective W deposition;

FIG. 4 is an SEM after deposition of the third step of a preferredembodiment of the present invention on a PFET; and

FIG. 5 is an SEM after deposition of the third step of a preferredembodiment of the present invention on an NFET.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed to a method and an integrated circuitproduced by the method for selectively depositing a refractory metallayer on exposed silicon surfaces of a silicon substrate utilizing athree step deposition process. Before the first step, the semiconductorwafer is preconditioned by a single step of a dilute hydrofluoric acid(DHF) dip to eliminate native oxides. The dip allows the wafer to beexposed to air for up to approximately 1 hour without oxidation.Heretofore, other selective-W methods have required a two steppreconditioning to achieve equivalent results which includes: 1) a DHFdip; and 2) a Si light etch (to remove contaminants in n+ and p+regions). This invention eliminates the need for the two steppreconditioning.

Referring to FIG. 1, preconditioned wafer 10 is placed inside arefractory gas CVD reactor 20 on top of pedestal heater 22 having anelectrical feedthrough 11. Gas is fed through the showerhead 24 throughthe gas sources 26, 28, 30, 32, 34. Backside gas 40 is also provided.The gas is exhausted through pump 50 by gate valve 52. Pressure iscontrolled using throttle valve 54. Similarly, backside gas 40 isexhausted through pump 60 through gate valve 62. Backside pressure iscontrolled using throttle valve 64.

FIG. 2 is an exemplary integrated circuit prior to selective Wdeposition. Integrated circuit 100 is processed by steps such asisolation formation of silicon dioxide spacers 101; doping of thesilicon substrate to form a p(+/-) substrate 102 and an n(+/-) substrate104; depositing gate oxide 103 to about 50-100 A, and the gate stack112, 114 to about 2000 A. The gate stack oxide is formed in and etchedinto the n(+/-) gate stack 112 and the p(+/-) gate stack 114. After thegate stacks 112, 114 are etched, side wall oxide 107 is depositedthereon followed by a nitride or oxide spacer 106. Nitride spacer isrelatively thin (approximately 100-500 A). After the chip has beenprocessed to the embodiment shown in FIG. 2, it is ready for the DHF dipand the three step process of selective deposition of the refractorymetal thereon.

During each of the selective-W deposition processing steps, sevenparameters are particularly crucial in obtaining the desired results.The seven parameters include: 1) temperature, 2) pressure, 3) time, 4)WF₆ flow, and 5) H₂ flow, 6) SiH₄ to WF₆ flow ratio, and 7) Ar gas flow.The total pressure is selected to be significantly greater than thepartial pressure of WF₆ and SiH₄.

Equipment used to deposit refractory metals may be divided into twogroups: 1) blanket W tools that operate at higher total pressures (above1 Torr and usually above 10 Torr); and 2) selective W tools that operateat "very" low total pressure (single mTorr). Heretofore, selective filmsof W with repeatability characteristics acceptable for commercialmanufacturing have only been obtainable at low total pressure. Blanket Whave usually been operated at high total pressure in manufacturingfacilities. Selective W tools are more expensive than blanket W toolsbecause: a) blanket tools are much more common as blanket deposition haswide acceptance in commercial manufacturing as compared to selectivedeposition; and b) selective tools are essentially blanket tools withadditional modifications that raise the base cost of the tool. Thisinvention has the unexpected advantages of successful use of blanket Wequipment to perform selective W deposition simultaneously on p and ntype doped silicon and of providing acceptable repeatability and deviceresults at high total pressure, unlike previous efforts in this field.One possible reason for this success is related to the difference ingrowth rates of films created at different total pressures. Thisinvention provides a selective W CVD process that operates in the WF₆mass transport (diffusion) limited regime thereby depositing Wselectively at a minimal rate and with dense nucleation resulting in asmoother microstructure and equal deposition over n⁺ and p⁺ regions(equal deposition in this context means that the W deposited on the p+region is within at least 50% of the thickness of W deposited on the n+region. For example, if the n+ region is 300 A then the p+ region is 150A).

In the first step, a refractory metal, such as W, is selectivelydeposited on the exposed surfaces of the silicon substrate by reacting arefractory metal halide, such as WF₆, with the exposed surfaces of saidsilicon substrate in the presence of an inert gas, such as Argon (Ar) inaccordance with the following: ##EQU5##

In the first step, the Si substrate is exposed to WF₆ at a flow rate of5-10 sccm and preferably set at 5 sccm; a temperature range of 275 to325° C. and at a preferred temperature of 300° C.; a partial pressurerange of 10-50 milliTorr (mTorr) for WF₆ and preferred pressure of 20mTorr; a total pressure range of 30,000 to 100,000 mTorr, with apreferred total pressure of 40,000 mTorr (40 Torr); a period of about 1second; a flow rate of zero for H₂ and a flow rate of 10,000-20,000standard cubic centimeters/minute (sccm) for Argon (Ar) with a preferredflow of 14000 sccm. The SiH₄ to WF₆ ratio is zero. The processing timeof step 1 is limited to 1 sec so that Si consumption and encroachmentare minimized. After about 1 second exposure to WF₆, 50-100 Agrowth/deposition on the p+ region have been observed.

Other literature has taught the need for a two step preconditioningsequence to equalize the growth of tungsten on n+ and p+ regions. Theliterature has taught the use of a dilute hydrofluoric acid (DHF) dip toremove surface oxide followed by a Si light etch (exposure to a reactiveion etch) to remove other surface contaminants. According to thisapproach of selective tungsten deposition, the Si light etch is integralin achieving deposition on the p+ region. The fact of absence of anygrowth on the p+ region even after a 60 sec exposure to WF₆ when the Silight etch is not used is provided as evidence of the importance of thisSi light etch step according to other practices of selective tungstenchemical vapor deposition.

This invention obviates the Si light etch step by growing tungsten inthe initial Si reduction phase under a WF₆ mass transfer limitedcondition (high total reactor pressure). Under this condition, thegrowth is limited by arrival rate of WF₆ at the p+ and n+ surfacesthereby reducing the sensitivity to surface effects such ascontamination not removed by the DHF dip. As a result, tungsten grows onboth p+ and n+ regions at substantially equal thickness.

The second step includes limiting silicon substrate consumption with therefractory metal halide while increasing the refractory metal thicknessby reacting the refractory metal halide with a silicon containing gas.##EQU6##

In the second step, SiH₄ is introduced. The gas of the second step isintroduced into the CVD reactor 20 quickly in order to avoid any furtherconsumption/encroachment of the Si substrate. In the second step, W isdeposited onto the silicon substrate with these parameter settings:substrate temperature in a range of 275 to 325° C. and preferably 300°C.; WF₆ partial pressure in the range of 10-50 mTorr for and preferablyat 20 mTorr; total pressure range in the range of 30,000 to 100,000mTorr, and preferably 40,000 mTorr (40 Torr); H₂ flow in the range of5-10 Standard Liters/Minute (SLM)and preferably 9 SLM; Ar flow in therange of 10-20 SLM and preferably 14 SLM, SiH₄ flow in the range of40-50 sccm and preferably 45 sccm; and WF₆ in the flow range of 5-10sccm and preferably 5 sccm. The SiH ₄ /WF₆ ratio during the second stepmay be as high as 10 because the time of second step is limited to 2-3sec so that the W added by step 2 is limited to about 50 A. Sicontamination in this layer does not increase the resistivity of thecontact layer because incorporated excess Si is consumed in step 3.

This invention also con templates the use of other sources of silane gassuch as an alkylsilane or disilane. In particular, alkylsilanes such asdiethylsilane, dimethylsilane, n-butylsilane, methylsilane, ethylsilanemay be used.

The purpose of step 2 is to further limit W growth by reaction of WF₆with substrate Si. W growth by reaction of WF₆ with substrate Si resultsin further substrate encroachment and consumption as Si atoms diffusefrom the substrate and react with WF₆ (as occurs in step 1). During thesilane reduction there are two reactions occurring. The first reactionis given by the silane reduction equation: ##EQU7## the second reactionbeing the reaction given by the WF₆ alone step equation: ##EQU8## Bothof the reactions may occur simultaneously. By controlling the flow ratioof SiH₄ to WF₆ and by having a sufficiently thick W layer from the firststep (50-100 A) to provide a diffusion barrier, the rates of the tworeactions, relative to each other are controlled. That is, as thesilane/WF₆ ratio becomes large, the silane reduction reactionpredominates. As the ratio approaches zero, the Si reduction reactionpredominates. Silane reduction deposits W while Si reduction W byconsuming substrate Si. If growth predominates there is substrateconsumption/encroachment but little Si contaminant within the film. Ifdeposition predominates there is Si contamination but little substrateconsumption/encroachment. This invention is motivated by minimizingsubstrate consumption which results in W deposition being deliberatelymore prevalent.

The time of step 2 is minimized to 2 to 3 seconds so as to reduce theroughness that this layer contributes to the final film, minimizeselectivity loss, and minimize resistivity. This invention includes ahigh SiH₄ to WF₆ ratio so that Si encroachment is prevented and theexcess Si incorporated in the thin SiH₄ reduced layer will provide Sithat may be consumed by the WF₆ present during the subsequent H₂reduction step thereby avoiding further Si encroachment.

The third step is for the purpose of further depositing the refractorymetal by reacting the refractory metal halide with hydrogen. ##EQU9##

For the third step W is deposited onto the silicon substrate with theseparameter settings: substrate temperature in a range of 275 to 325° C.and preferably 300° C.; WF₆ partial pressure in a range of 10-50milliTorr (mTorr) and preferably 20 mTorr; total pressure in a range of30,000 to 100,000 mTorr and preferably 40,000 mTorr (40 Torr); H₂ flowin the range of 5-10 Standard Liters/Minute (SLM) and preferably 9 SLMfor H₂ ; Ar flow in the range of 10-20 SLM and preferably 14 SLM; andWF₆ in the range of 5-10 sccm and preferably 5 sccm. To limitselectivity loss (deposition of W on oxide), silane is turned off in thethird step. The time for the third step is about 3-5 seconds to provideabout an additional 100 A W. Times greater than 5 sec. may be selectedfor the third step to thicken the W layer to 300-400 A withoutselectivity loss depending on device requirements.

The times of the first and second steps are selected such that about 100A of W has been deposited by the end of the second step (at which timeSiH₄ is turned off). The 100 A of W provides an adequate barrier todiffusion of Si. Thus, the third step does not result in further Siconsumption or encroachment. Other advantages of the third step vs. thesecond step are deposition of a smoother film, fewer paths for loss ofselectivity, and elimination of excess Si that may deposit from the SiH₄reduction reaction in the second step.

FIG. 4 is a Scanning Electron Micrograph (SEM) of a the Source/Drain(S/D) region of a p+ field effect transistor (PFET) following the thirddeposition step. FIG. 5 is an SEM of the Source/Drain region of an NFETfollowing the third deposition step.

The morphology of the PFET and NFET regions are relatively smooth afterdeposition. The smooth surface provides more effective results duringthe post processing steps. For example, the device will have improvedcontact resistance, minimizing device failure. In addition, alignmentfor photolithography is more readily achieved with a smoother surface.The surrounding oxide regions do not show any W deposition.

In FIG. 3, the integrated circuit device 200 is illustrated followingselective W deposition. The W deposition is shown at the 202. Using thetimes for each of the three steps above, the total W thickness willrange between about 200-300 A. The W deposition 202 covers the n+ gatestack 112 and p+ gate stack 114 as well as the lightly drained implants108 and 110.

Prophetically, the refractory metal Mo may also be used in a three stepprocess. The processing steps for Mo would be performed at parameterssimilar to W and would include the steps of: ##EQU10##

This invention adequately addresses the difficulties of uneven n+ versusp+ (source/drain) growth, deep consumption/encroachment by therefractory metal into silicon regions (e.g., worm holes), poor adhesion,uncontrolled selectivity and uneven morphology.

EXAMPLE

This example describes the deposition of W on a Si substrate. A 200:1DHF dip was used to remove 150 A of native oxide. Commercially availableblanket CVD W equipment was used to deposit W on a substrate withexposed silicon areas in a mostly oxide field. At 300° C. perform thesequence: 1 sec. Si reduction of WF₆, 3 sec. SiH₄ reduction of WF₆, and5 sec. H₂ reduction of WF₆ to deposit about 200-300 A W over exposed Siareas. Excellent selectivity was achieved on the center and edge of thewafer. Equal deposition was achieved over the n and p regions and the Wfilms did not peel after anneal at 500° C. for 30 min. No specialhardware modifications to the commercially available blanket CVD Wreactor were required and a vacuum integrated sequence to clean the Sisurface before the onset of W deposition was not required.

What is claimed is:
 1. A silicon substrate comprising:a p dopedsubstrate region having an n Field Effect Transistor (NFET) thereon; andan n doped substrate region having a p Field Effect Transistor (PFET)thereon, wherein said NFET and said PFET each have depositions of arefractory metal less than 300 Angstroms deposited thereon.
 2. Thesubstrate of claim 1, wherein the refractory metal is tungsten.
 3. Thesilicon substrate of claim 1, wherein the depositions on the p regionand the n region are substantially equal.